Demand for clean energy fuels such as hydrogen, methanol, liquid fuels (GTL: Gas to Liquids), and dimethyl ether (DME) is expected to significantly increase in the future. Therefore, a reforming apparatus for producing such a synthetic gas tends to be large in size, and an apparatus that achieves higher thermal efficiency and is suitable for mass production is demanded. Also, heat exchange for recovering exhaust is often used to enhance energy efficiency in reforming apparatuses in the conventional petroleum refining, petrochemical plants, and the like, and ammonia manufacturing apparatuses, hydrogen manufacturing apparatuses, and the like, in which raw materials such as petroleum are used.
To effectively use the heat of such a high-temperature gas, heat exchange in a temperature range of 400 to 800° C., which is relatively low, has become important, and corrosion caused by carburization of a high Cr-high Ni—Fe alloy based metal material used for reaction tubes, heat exchangers, and the like in this temperature range poses a problem.
Usually, a synthetic gas reformed in the above-described reactors, that is, a gas containing H2, CO, CO2, H2O, and hydrocarbon such as methane comes into contact with the metal material of a reaction tube and the like at a temperature of about 1000° C. or higher. In this temperature range, on the surface of the metal material, elements such as Cr and Si, which have higher oxidation tendency than Fe or Ni or the like, are oxidized selectively, and a dense film of chromium oxide or silicon oxide or the like is formed, by which corrosion is restrained. In a portion such as a heat exchange part in which the temperature is relatively low, however, the diffusion of element from the inside to the surface of metal material is insufficient. Therefore, the formation of oxide film, which achieves a corrosion restraining effect, delays, and additionally, such a gas having a composition containing hydrocarbon comes to have carburizing properties, so that carbon intrudes into the metal material through the surface thereof, and carburization occurs.
In an ethylene cracking furnace tube and the like, if carburization proceeds and a carburized layer comprising carbide of Cr or Fe or the like is formed, the volume of that portion increases. As a result, fine cracks are liable to develop, and in the worst case, the tube in use is broken. Also, if the metal surface is exposed, carbon precipitation (coking) in which metal serves as a catalyst occurs on the surface, so that the flow path area of the tube decreases and the heat-transfer characteristics degrade.
In a heating furnace tube and the like for a catalytic cracking furnace for increasing the octane value of naphtha obtained by distillation of crude oil as well, a heavily carburizing environment consisting of hydrocarbon and hydrogen is created, so that carburization and metal dusting occur.
On the other hand, in an environment in which the carburizing properties of gas in the reforming furnace tube, heat exchanger, and the like are severer, the carbide is supersaturated, and thereafter graphite precipitates directly. Therefore, a base material metal is exfoliated away and the thickness of base material decreases, that is, corrosion loss called metal dusting proceeds. Further, coking occurs with the exfoliated metal powder serving as a catalyst.
If the cracks, loss, and in-tube closure increase, an apparatus failure or the like occurs. As a result, operation may be suspended. Therefore, careful consideration must be given to the selection of material used for an apparatus member.
To prevent the aforementioned carburization and the corrosion caused by metal dusting, various countermeasures have conventionally been studied.
For example, Patent Document 1 proposes an Fe-based alloy or a Ni-based alloy containing 11 to 60% (mass %, the same shall apply hereinafter) of Cr concerning the metal dusting resistance in an atmospheric gas of 400 to 700° C. containing H2, CO, CO2 and H2O. Specifically, it is shown that the invention of an Fe-based alloy containing 24% or more of Cr and 35% or more of Ni, a Ni-based alloy containing 20% or more of Cr and 60% or more of Ni, and an alloy material in which Nb is further added to these alloys is excellent. However, even if a Cr or Ni content in the Fe-based alloy or the Ni-based alloy is merely increased, a sufficient carburization restraining effect cannot be achieved, so that a metal material having higher metal dusting resistance has been demanded.
Also, in a method disclosed in Patent Document 2, to prevent corrosion caused by metal dusting of a high-temperature alloy containing iron, nickel, and chromium, one or more kinds of metals of the VIII group, the IB group, the IV group, and the V group of the element periodic table and a mixture thereof are adhered to the surface by the ordinary physical or chemical means, and the alloy is annealed in an inert atmosphere to form a thin layer having a thickness of 0.01 to 10 μm, by which the alloy surface is protected. In this case, Sn, Pb, Bi, and the like are especially effective. Although effective at the early stage, this method may lose effectiveness in that the thin layer is exfoliated in long-term use.
Patent Document 3 relates to the metal dusting resistance of a metal material in an atmospheric gas of 400 to 700° C. containing H2, CO, CO2 and H2O. As the result of an investigation of the interaction with carbon made from the viewpoint of solute element in iron, Patent Document 3 discloses that the addition of an element producing stable carbide in the metal material, such as Ti, Nb, V and Mo, or the alloying element in which the interaction co-factor Ω represents a positive value, such as Si, Al, Ni, Cu and Co is effective in restraining metal dusting in addition to enhancing the protecting properties of oxide film. However, the increase of Si, Al and the like sometimes leads to the decrease in hot workability and weldability. Therefore, considering the manufacturing stability and plant working, this metal material leaves room for improvement.
Next, to break off the contact of carburizing gas with the metal surface, there have been disclosed a method for oxidizing a metal material in advance and a method for performing surface treatment.
For example, Patent Document 4 and Patent Document 5 disclose a method for pre-oxidizing a low Si-based 25Cr-20Ni (HK40) heat resistant steel or a low Si-based 25Cr-35Ni heat-resisting steel at a temperature near 1000° C. for 100 hours or longer in the air. Also, Patent Document 6 discloses a method for pre-oxidizing an austenitic heat-resisting steel containing 20 to 35% of Cr in the air. Further, Patent Document 7 proposes a method for improving the carburization resistance by heating a high Ni—Cr alloy in a vacuum and by forming a scale film.
Patent Document 8 proposes an austenitic alloy whose contents of Si, Cr and Ni satisfy the formula of Si<(Cr+0.15Ni−18)/10; thereby a Cr-based oxide film having high adhesiveness even in an environment, in which the alloy is subjected to a heating/cooling cycle, is formed to provide the alloy with excellent carburization resistance even in an environment in which the alloy is exposed to a corrosive gas at high temperatures. Patent Document 9 proposes an austenitic stainless steel having excellent scale exfoliation resistance even in an environment in which the steel is subjected to a heating/cooling cycle, which is produced by containing Cu and a rare earth element (Y and Ln group) therein and thereby forming a uniform oxide film having high Cr concentration in the film. In this patent document, however, the influence of Cu addition on the weldability or the creep ductility has not been studied. Patent Document 10 proposes a method for improving the carburization resistance by forming a concentrated layer of Si or Cr by performing surface treatment. Unfortunately, all of these prior arts require special heat treatment or surface treatment, and therefore they are inferior in economy. Also, since scale restoration (scale recycling) after the pre-oxidized scale or the surface treatment layer has exfoliated away is not considered, if the material surface is damaged once, the subsequent effect cannot be anticipated.
Patent Document 11 proposes a stainless steel pipe having excellent carburization resistance and containing 20 to 55% of Cr, which is produced by forming a Cr-deficient layer, which has a Cr concentration of 10% or higher and lower than the Cr concentration of the base material, on the surface of steel pipe. In this patent document, however, improvement has not been made at all on the decrease in weldability caused by containing Cr or the addition of Si. Also, Patent Document 12 proposes a metal material in which the HAZ crack susceptibility, which is one property of weldability, is decreased by increasing the content of C of an Si and Cu containing steel. This patent document, however, does not provide a drastic solution because the high C content increases the weld solidification crack susceptibility, and also decreases the creep ductility.
Besides, a method for adding H2S into the atmospheric gas has been thought of. However, the application of this method is restricted because H2S may remarkably decrease the activity of a catalyst used for reforming.
Patent Document 13 and Patent Document 14 propose a metal material in which the gas dissociative adsorption (gas/metal surface reaction) is restrained by containing a proper amount of one kind or more kinds of P, S, Sb and Bi. Since these elements segregate on the metal surface, even if the elements are not added excessively, the elements can restrain carburization and metal dusting corrosion significantly. However, since these elements segregate not only on the metal surface but also at the grain boundary of metal grainy, a problem associated with hot workability and weldability remains to be solved.
Techniques for enhancing corrosion resistance and crevice corrosion resistance by adding Cu have also been proposed. Patent Document 15 describes a technique for enhancing corrosion resistance by containing Cu, and on the other hand, for increasing the hot workability improving effect due to B by reducing S and O as far as possible. Patent Document 16 describes a technique for improving corrosion resistance and crevice corrosion resistance excellent in sulfuric acid and sulfate environments by setting the G.I. value (General Corrosion Index) represented by “—Cr+3.6Ni+4.7Mo+11.5Cu” at 60 to 90 and by setting the C.I. value (Crevice Corrosion Index) represented by “Cr+0.4Ni+2.7Mo+Cu+18.7N” at 35 to 50. Patent Document 17 describes a technique for improving hot workability by adding B exceeding 0.0015% while increasing a Cu content and by keeping an oxygen content low. In all of these techniques, the upper limit of a C content is restricted to a low level to avoid the decrease in corrosion resistance. Therefore, the solid-solution strengthening of C cannot be anticipated, and a sufficient high-temperature strength cannot be obtained. For this reason, these techniques are unsuitable for a metal material used at high temperatures.